1. Field of the Invention
The invention lies in the field of optoelectronic packages or modules, in particular the field of plastics material packages. It relates to a method of fixing an optoelectronic component support base included in the package.
2. Technological Background
An optoelectronic module comprises a package from which emerge optical and electrical connections for connecting the package to other modules, for example, or to a structure accommodating a set of modules.
The package accommodates a base, for example made of silicon, Kovar or possibly another metal. If the package is of metal, for example, the base can be welded or glued to the package. The base has a surface fixed to the bottom of the package or to a connector or to the bottom of the package and to a connector. The base can include silica surface portions, the silica portion accommodating portions forming a waveguide for a light wave. The base can include grooves, for example V-shaped or U-shaped grooves, etched into the surface and whose function is to facilitate the positioning of an optical fiber.
Optoelectronic components, for example, a laser diode and possibly other optical items such as lenses are mounted on one or more faces of the base, for example on a face of the base opposite the face disposed against the bottom of the package. Hereinafter the expression xe2x80x9coptical componentxe2x80x9d will refer to any optical component, whether it is an active component such as a laser or a passive component such as a coupling lens or a waveguide, in particular a silica waveguide integrated into the base, as indicated above. The optical connection of an optical component is effected by means of a connector integrated into the package. The connector receives one end of an optical fiber. The other end of the fiber faces the optical component, with the result that an optical signal can pass from the fiber to the component or from the component to the fiber.
The assembly sequence for a module necessitates fixing the fiber at both ends. For a plastics material module, the fiber is first fixed with an adhesive into a groove in the base, for example a V-shaped groove, facing an optical component. The other end of the fiber is then fixed into the connector and at the same time the base is fixed to a bottom of the package or to the connector or to the bottom of the package and the connector, using the same adhesive.
The fiber is subjected to shocks or thermal cycling during the assembly of the various portions of the package and during the use of the module. It follows that expansion stresses are induced in the fiber because of the different coefficients of thermal expansion of the various portions constituting the equipped package assembly.
These stresses on the fiber can cause displacements of the ends of the fiber, in particular of the end facing the optical component, and thus a loss of optical coupling between the fiber and the optical component. As a general rule, in optoelectronic packages, the fiber length is of the order of ten centimeters. With a length of this magnitude, it is possible to provide a curvature to absorb differential expansion.
On the other hand, in technologies using surface mounting of components (SMT), which generally employ more compact packages, the fiber length is of the order of 2 cm. In this case it is no longer possible to use a curvature to absorb differential expansion. It is therefore necessary to find some other assembly solution so that the ends of the fiber remain at their nominal location despite the stresses of thermal origin, and this applies in particular to the end facing the optical component.
According to the inventors, the source of the stress exerted on the fiber, leading to displacement of the end facing, for example, the optical component and possibly the other end, originates essentially from the differential expansion of, on the one hand, the silica fiber and, on the other hand, other items, such as the bottom of the package, the connector, and the base, made of plastics material, silicon, Kovar, or another metal. The expansion of the package, the connector or the base applies a tension stress to the fiber. This stress can lead to displacement of at least one end of the fiber, and in particular of the end facing the optoelectronic component. The positioning tolerances for good optical coupling of that end are generally closer than the positioning tolerances for the end connected to the connector.
In the method of assembling a module in accordance with the invention the two ends of the fiber are fixed to the base and to the connector in a manner that is less deformable than the connection of the base to the bottom of the package or to the connector or to the bottom of the package and to the connector. This relieves the stress of thermal origin exerted on the fiber, enabling the base to move relative to the items to which it is itself fixed, for example the bottom of the package, the connector or both.
To obtain this result, it is sufficient for the adhesive connecting the base and the items to which it is itself fixed to be more elastic than the adhesive used to fix at least one end and preferably both ends of the fiber. This means that the elongation at the threshold of plasticity of the adhesive connecting the base and the items to which it is itself fixed is higher than the elongation at the threshold of plasticity of the adhesive fixing at least one end of the fiber.
To summarize, the invention relates to an optical module including a package having a bottom and accommodating:
a base on which is mounted an optical component, and
an optical connector,
the connector and the component being coupled by an optical fiber having two ends of which one end is fixed by an adhesive to the base in front of the optical component and the other end is fixed by an adhesive to the connector, the base being fixed to the bottom of the package or to the connector or to the bottom of the package and to the connector by an adhesive, which module is characterized in that the adhesive for fixing the base has an elongation at the threshold of plasticity greater than the elongation at the threshold of plasticity of the adhesive for fixing at least one end of the fiber to the base or the connector.
With this feature, the adhesive or adhesives fixing the base are deformed more for the same stress than the adhesive or adhesives fixing the ends of the fiber. As a result the base moves for a lower level of stress than that which causes the ends of the fiber to move, which relieves the stress on the fiber.
The deformation at the threshold of plastic deformation of the adhesive connecting the base is preferably from 10 to 100 times greater than that of the adhesive fixing the fiber.
As a general rule, this will imply that the Young""s modulus of the adhesive fixing the base is very much less than, for example less than one third of, the Young""s modulus of the adhesive fixing the ends of the optical fiber.
Of course, the fact that the connector and the base are coupled by an optical fiber does not exclude the situation in which the connector and the base are coupled by several optical fibers, for example 4, 8 or 16 optical fibers. Similarly, the package can include several bases or several connectors.